The present invention relates to a fired heater for heating process fluids, e.g., process heaters. More specifically, it relates to a fired heater of the type which comprises at least one radiant section in which process fluid flowing through tubes therein is indirectly heated, preferably, by radiant energy provided by burners. The methods and apparatus used in accordance with the present invention are particularly well suited and advantageous for pyrolysis of normally liquid or normally gaseous aromatic and/or aliphatic hydrocarbon feedstocks such as ethane, propane, naphtha or gas oil to produce ethylene and other by products such as acetylene, propylene, butadiene, etc. Accordingly, the present invention will be described and explained in the context of hydrocarbon pyrolysis, particularly steam cracking to produce ethylene.
Steam-cracking is the predominant commercial method for producing-light olefins such as ethylene, propylene and butadiene. Ethylene, propylene, and butadiene are basic building block chemicals used in the manufacture of high volume polymeric materials and commercially important chemical intermediates. The demand for these basic building block petrochemicals is expected to continue to grow in the foreseeable future. Of the products produced in steam cracking, ethylene has the highest demand, and is the most costly to separate and purify. Therefore improving the yield or selectivity of ethylene is highly desired. Steam cracking involves a thermal cracking reaction typically carried out in a fired tubular reactor. Reactor selectivity to ethylene is favored by short residence time and low hydrocarbon partial pressures. Hydrocarbon feeds ranging from ethane to vacuum gas-oil are used, and the reaction is conducted in the presence of dilution steam. The complex reactions and the tubular reactor are extensively discussed in both public domain literature and numerous patents.
Steam cracking of hydrocarbons has typically been effected by supplying the feedstock in vaporized or substantially vaporized form, in admixture with substantial amounts of dilution steam, to suitable coils in a cracking furnace. It is conventional to pass the reaction mixture through a number of parallel coils or tubes which pass through a convection section of the cracking furnace wherein hot combustion gases raise the temperature of the feed and dilution steam. Each coil or tube then passes through a radiant section of the cracking furnace wherein a multiplicity of burners supply the heat necessary to bring the reactants to the desired reaction temperature and effect the desired reaction.
Of primary concern in all steam cracking process configurations is the formation of coke. When hydrocarbon feedstocks are subjected to the heating conditions prevalent in a steam cracking furnace, coke deposits tend to form on the inner walls of the tubular members forming the cracking coils. Such coke deposits interfere with heat flow through the tube walls into the stream of reactants, which results in higher tube metal temperatures, ultimately reaching the limits of the tube metallurgy. Additionally, the coke deposits interfere with the flow of the reaction mixture resulting in higher pressure drop, due to reduced tube cross sectional area.
The optimum way of improving selectivity to ethylene was found to be by reducing coil volume while maintaining the heat transfer surface area. This has been accomplished by replacing large diameter, serpentine coils with a multiplicity of smaller diameter tubes having a greater surface-to-volume ratio than the large diameter tubes. The tubes typically have inside diameters up to about 7.6 cm (3 inches), generally from about 3.0 cm to 6.4 cm (1.2 to 2.5 inches).
The desire for short residence times has led to the use of shorter coils, typical lengths being progressively reduced over the years from over 45 m (150 ft.) to 20 m-27 m (60-90 ft.) and more recently 9 m-12 m (30-40 ft). As coils have been reduced in length, it has been necessary to reduce the tube diameter in an effort to reduce the heat flux and hence the tube metal temperatures. Current cracking coils are generally constructed from high alloy (25% Cr, 35% Ni, plus additives) austenitic stainless steels, and are operated at maximum tube metal temperatures in the range 1030-1150 degrees C. (1900-2100 degrees F.).
Despite the significant evolution of cracking furnace design, the process is still limited by the fact that it makes as a byproduct coke, which deposits on the inside of the coils. The coke acts as an insulator, and hence increases the tube metal temperatures of the coil. When the tube metal temperature reaches the maximum capability of the material it is necessary to cease production and decoke the furnace. This is generally carried out by passing a mixture comprising air and steam through the coils at high temperature. The coke is removed by a combination of combustion and erosion/spalling. Other decoking techniques which avoid the use of air are also used in the industry. In this case the coke is removed primarily by erosion/spalling and gasification. Regardless of the decoking technique that is used some of the spalled coke is in the form of large particles. As tube diameters have decreased the likelihood of large coke particles plugging the coil before or during decoking have increased. Decoking typically takes from 12-48 hours, depending on a variety of factors including: the furnace design, the feed that was cracked, the operating time before the decoke, and the cracking severity employed.
Technology to reduce tube metal temperatures (and hence coking rates, or alternatively to allow a shorter residence time coil to be used) has been widely sought by the industry. Some designers have resorted to multiple inlet leg coils to reduce the heat flux on outlet tubes (e.g.; EP 0 305 799 A1). Others have attempted to prevent the formation of the insulating coke layer inside the tube by adding small concentrations of specific elements to the reactor feed.
Heat transfer to the highly endothermic cracking reaction may be represented by the familiar equation Q=Uxc3x97Axc3x97xcex94T. U, the heat transfer coefficient is a function of the gas velocity inside the tube. Higher velocities increase U, and hence reduce the required xcex94T (temperature difference) thus reducing tube metal temperature for a given process fluid temperature. However, as velocities increase, pressure drop increases, increasing the coil average hydrocarbon partial pressure. Eventually the pressure effect over-rides the effect of reduced residence time, and further increases in velocity reduce reactor selectivity to ethylene. This represents a maximum practical value to U.
Overall area (A) may be increased by using multiple small diameter tubes. This trend has been pursued by the industry, resulting in reactors with tubes of inside diameter 2.5 cm-3.8 cm (1.0-1.5 inch). This represents a minimum practical diameter due to manufacturing limitations, and below these diameters the effects of coke build-up inside the tube becomes excessive.
The general principle of increasing internal surface area to improve heat transfer is well known in the general heat transfer art. However, applying this principle to very high temperature coking services like steam cracking is difficult.
Nevertheless, this method of improving heat transfer to reduce the tube metal temperatures in steam cracking furnaces has been proposed in several varieties. One example (U.S. Pat. No. 4,342,242) uses a specially designed longitudinal insert in an otherwise circular tube cross section. The insert has a central body and outwardly extending vanes which contact the interior of the coil. In this particular disclosure the insert is positioned in only a portion of the overall tubular coil in the furnace. Another example (GB 969,796) utilizes internally rounded channels or fins which enhance the inside area. The internal profile was smooth to avoid stress concentrations and flow disturbances. The specific tubes described in this disclosure made 4 passes through the radiant section and had a relatively large 9.525 cm (3.75 inch) inside diameter.
Variations of this rounded internal channels or finned tube profile have been applied commercially in specific coil designs. A paper presented at an American Institute of Chemical Engineers Meeting (xe2x80x9cSpecialty Furnace Design Steam Reformers and Steam Crackersxe2x80x9d by T. A. Wells, presented at the 1988 AlChE Spring National Meeting, New Orleans, La., Mar. 6-10, 1988) discloses the use of a type of extended internal surface tube in a single tube pass design. The inlet legs of longer coils (EP 0 305 799 A1) and a literature reference for this design, denominated SRT V (Energy Progress Vol. 8, No. 3, p. 160-168 , September 1988) have utilized internal extended surface. In both of the latter cases the commercial use was based on tubes of approximately 2.5-3.8 cm (1.0-1.5 inch) inside diameter and where the tube section that had the rounded internal channels or fins made only a single pass through the furnace radiant section. Another literature reference (xe2x80x9cUSC Super U Pyrolysis Conceptxe2x80x9d by David J. Brown, John R. Brewer and Colin P. Bowen presented at the AlChE Spring National Meeting in Orlando, Fla., March 1990) presents data on tubes with internal fins on the inlet leg. This reference speculates that providing fins on the outlet leg would be beneficial, however it provides no suggestion as to what operating or design parameters would be required to successfully demonstrate or enable the use of fins on the outlet leg.
However, an extended internal surface design to this time, has not been shown to be feasible in two pass coils typically made up of U shaped tubes. These two pass coils are typically 15 m-27 m (50-90 ft.) in total length, with internal diameters in the range 3.8 cm-6.4 cm (1.5-2.5 inch). Two pass coils can be as short as 13 m (40 ft). One problem is that the capability to make an internally finned tube long enough to form the complete U shaped tube does not exist.
An internally finned tube could be used just for the inlet half of the U shaped tubes, as described in EP 0 305 799 A1 which uses internal fins, studs or inserts only on the inlet tubes to the furnacexe2x80x94not the outlet. This reference discloses that inserts located in the outlet tube would be expected to act as nucleus for the growth of the coke formed during pyrolysis. However, the highest tube metal temperatures occur near the outlet end, so the advantageous effect of the finned tube is not applied to where it is most needed. Applying the finned tube to the outlet leg of the coil would be possible, but it carries the risk that coke pieces from the inlet leg could break loose and become lodged at the start of the finned section. Finally, the industry conventional wisdom suggested that a bent finned tube section would be prone to plugging with coke spalled from the inlet leg of the coil.
In light of the known deficiencies in heat transfer in steam cracking furnaces there is a need for a means to increase the heat transfer in the inside of the tubes to reduce coking, tube metal temperature and improve ethylene selectivity. Particularly, it would be highly desirable to have a design for a 2 pass coil or U shaped tubes that uses some means of increased internal surface area to reduce tube metal temperature throughout its entire length.
The present invention is directed towards a fired heater for heating a process fluid that provides increased internal heat transfer surface to reduce tube metal temperatures at the inlet and outlet of a U shaped tube and at the same time is not prone to plugging from coke. The fired heater comprises a radiant section enclosure having a plurality of U shaped tubes disposed therein, an inlet for introducing the process fluid into the U shaped tubes, burners for exposing the external surface of the U shaped tubes to radiant heat, an outlet for cooling and collecting the process fluid from the U shaped tubes, wherein the U shaped tubes are formed by connecting one or more tubular sections; and at least the outlet leg of the U shaped tubes are provided with internal generally longitudinal fins. In another embodiment the entire length of the U shaped tubes are provided with internal generally longitudinal fins.